In 1969-1970, Kenneth Thompson, Dennis Ritchie, and others at
AT&T Bell Labs began developing
a small operating system on a little-used PDP-7.
The operating system was soon christened Unix, a pun on an earlier operating
system project called MULTICS.
In 1972-1973 the system was rewritten in the programming language C,
an unusual step that was visionary: due to this decision, Unix was
the first widely-used operating system that
could switch from and outlive its original hardware.
Other innovations were added to Unix as well, in part due to synergies
between Bell Labs and the academic community.
In 1979, the “seventh edition” (V7) version
of Unix was released, the grandfather of all extant Unix systems.

After this point, the history of Unix becomes somewhat convoluted.
The academic community, led by Berkeley, developed a variant called the
Berkeley Software Distribution (BSD), while AT&T continued developing
Unix under the names “System III” and later “System V”.
In the late 1980’s through early 1990’s
the “wars” between these two major strains raged.
After many years each variant adopted many of the key features of the other.
Commercially, System V won the “standards wars”
(getting most of its
interfaces into the formal standards), and
most hardware vendors switched to AT&T’s System V.
However, System V ended up incorporating many BSD innovations, so the
resulting system was more a merger of the two branches.
The BSD branch did not die, but instead became widely used
for research, for PC hardware, and for
single-purpose servers (e.g., many web sites use a BSD derivative).

In 1984 Richard Stallman’s Free Software Foundation (FSF) began the GNU
project, a project to create a free version of the Unix operating system.
By free, Stallman meant software that could be freely
used, read, modified, and redistributed.
The FSF successfully built a vast number of
useful components, including a C compiler (gcc), an
impressive text editor (emacs), and a host of fundamental tools.
However, in the 1990’s the FSF
was having trouble developing the operating system kernel [FSF 1998];
without a kernel their dream of a completely free operating system
would not be realized.

In 1991 Linus Torvalds began developing an operating system kernel, which
he named “Linux” [Torvalds 1999].
This kernel could be combined with the FSF material and other components
(in particular some of the BSD components and MIT’s X-windows software) to
produce a freely-modifiable and very useful operating system.
This book will term the kernel itself the “Linux kernel” and
an entire combination as “Linux”.
Note that many use the term “GNU/Linux” instead for this combination.

In the Linux community,
different organizations have combined the available components differently.
Each combination is called a “distribution”, and the organizations that
develop distributions are called “distributors”.
Common distributions include Red Hat, Mandrake, SuSE, Caldera, Corel,
and Debian.
There are differences between the various distributions,
but all distributions are based on the same foundation: the
Linux kernel and the GNU glibc libraries.
Since both are covered by “copyleft” style licenses, changes to
these foundations generally must be made available to all, a
unifying force between the Linux distributions at their foundation
that does not exist between the BSD and AT&T-derived Unix systems.
This book is not specific to any Linux distribution; when it
discusses Linux it presumes Linux
kernel version 2.2 or greater and the C library glibc 2.1 or greater,
valid assumptions for essentially all current major
Linux distributions.

Increased interest in software that is freely shared
has made it increasingly necessary to define and explain it.
A widely used term is “open source software”, which is further defined in
[OSI 1999].
Eric Raymond [1997, 1998] wrote several seminal articles examining
its various development processes.
Another widely-used term is “free software”, where the “free”
is short for “freedom”:
the usual explanation is “free speech, not free beer.”
Neither phrase is perfect.
The term
“free software” is often confused with programs whose executables are
given away at no charge, but whose source code cannot be viewed, modified,
or redistributed.
Conversely, the term “open source” is sometime (ab)used
to mean software whose
source code is visible, but for which there are limitations on
use, modification, or redistribution.
This book uses the term “open source” for its usual meaning, that
is, software which has its source code freely available for
use, viewing, modification, and redistribution; a more detailed
definition is contained in the
Open Source Definition.
In some cases, a difference in motive is suggested;
those preferring the term “free software” wish to strongly
emphasize the need for freedom, while those using the term may have
other motives (e.g., higher reliability) or simply wish to appear less
strident.
For information on this definition of free software, and
the motivations behind it, can be found at
http://www.fsf.org.

Those interested in reading advocacy pieces for open source software
and free software should see
http://www.opensource.org and
http://www.fsf.org.
There are other documents which examine such software, for example,
Miller [1995]
found that the open source software were noticeably
more reliable than proprietary software
(using their measurement technique, which measured
resistance to crashing due to random input).

This book uses the term “Unix-like” to describe
systems intentionally like Unix.
In particular, the term “Unix-like” includes
all major Unix variants and Linux distributions.
Note that many people simply use the term “Unix” to describe these systems
instead.
Originally, the term “Unix” meant a particular product developed
by AT&T.
Today, the Open Group owns the Unix trademark, and it defines Unix as
“the worldwide Single UNIX Specification”.

Linux is not derived from Unix source code, but its interfaces are
intentionally like Unix.
Therefore, Unix lessons learned generally apply to both, including information
on security.
Most of the information in this book applies to any Unix-like system.
Linux-specific information has been intentionally added to
enable those using Linux to take advantage of Linux’s capabilities.

Unix-like systems share a number of security mechanisms, though there
are subtle differences and not all systems have all mechanisms available.
All include user and group ids (uids and gids) for each process and
a filesystem with read, write, and execute permissions (for user, group, and
other).
See Thompson [1974] and Bach [1986]
for general information on Unix systems, including their basic
security mechanisms.
Chapter 3
summarizes key security features of Unix and Linux.